Bridge fiber, combiner, and fiber laser device

ABSTRACT

A bridge fiber includes a core layer  31  and an outer layer  32  which has an index of refraction higher than that of the core layer  31  and covers the outer peripheral surface of the core layer  31 . The outer layer  32  is surrounded by a substance such as the atmosphere having an index of refraction lower than an index of refraction n 2  of the outer layer  32 . An area AR 1  of the outer layer  32  at one end face of the bridge fiber is an area that is to be optically coupled to an end face of a core of each of a plurality of pumping light inputting optical fibers, while an area AR 2  of the core layer  31  at another end face of the bridge fiber is an area that is to be optically coupled to an end face of a core of an amplification optical fiber  40.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a bridge fiber, a combiner, and a fiberlaser device that are suited for reducing light returning to a pumpinglight source from an amplification optical fiber.

2. Description of Related Art

There has been implemented a fiber laser device in which pumping lightemitted from a pumping light source enters an amplification opticalfiber through a combiner. In some cases, the fiber laser device of thiskind experiences failure of the pumping light source when the lightamplified by the amplification optical fiber returns to the combiner asreturn light and is propagated to the pumping light source through thecombiner.

Patent Literature 1 has been disclosed as a measure to remove suchreturn light. In Patent Literature 1, an anti-reflective film isattached at an entrance end of the amplification optical fiber in orderto guide the light to the outside, the light being generated within theamplification optical fiber and having a specific wavelength.

CITATION LIST Patent Literature

Patent Literature 1: JP-A-5-82914

SUMMARY OF THE INVENTION

It is however difficult in the method described in Patent Literature 1to remove the return light having a wavelength other than the specificwavelength, whereby the return light may cause failure in the pumpinglight source in some cases. Moreover, in the method described in PatentLiterature 1, the anti-reflective film is interposed between a bridgefiber included in a combiner and the amplification optical fiber,thereby creating a space between the bridge fiber and the amplificationoptical fiber, which tends to complicate the alignment of an opticalaxis.

Accordingly, an object of the present invention is to provide a bridgefiber, a combiner, and a fiber laser device that can easily reducefailure of a pumping light source caused by return light.

In order to achieve such object, according to a first aspect of thepresent invention, there is provided a bridge fiber including: a corelayer; and an outer layer which has an index of refraction higher thanan index of refraction of the core layer and covers an outer peripheralsurface of the core layer, wherein the outer layer is surrounded by asubstance having an index of refraction lower than the index ofrefraction of the outer layer, an area of the outer layer at one endface of the bridge fiber is an area that is to be optically coupled toan end face of a core of each of a plurality of pumping light inputtingoptical fibers, and an area of the core layer at another end face of thebridge fiber is an area that is to be optically coupled to an end faceof a core of an amplification optical fiber.

Further, according to a second aspect of the present invention, there isprovided a combiner including: a plurality of pumping light inputtingoptical fibers; and a bridge fiber including: a core layer; and an outerlayer which has an index of refraction higher than an index ofrefraction of the core layer and covers an outer peripheral surface ofthe core layer, wherein the outer layer is surrounded by a substancehaving an index of refraction lower than the index of refraction of theouter layer, an area of the outer layer at one end face of the bridgefiber is optically coupled to an end face of each of the plurality ofpumping light inputting optical fibers, and an area of the core layer atanother end face of the bridge fiber is an area that is to be opticallycoupled to an end face of a core of an amplification optical fiber.

Further, according to a third aspect of the present invention, there isprovided a fiber laser device including: a plurality of pumping lightinputting optical fibers; an amplification optical fiber; a bridge fiberincluding: a core layer; and an outer layer which has an index ofrefraction higher than an index of refraction of the core layer andcovers an outer peripheral surface of the core layer; and a pair ofmirrors which is disposed at a predetermined distance from each other inthe amplification optical fiber, wherein the outer layer is surroundedby a substance having an index of refraction lower than the index ofrefraction of the outer layer, an area of the outer layer at one endface of the bridge fiber is optically coupled to an end face of each ofthe plurality of pumping light inputting optical fibers, and an area ofthe core layer at another end face of the bridge fiber is opticallycoupled to an end face of a core of the amplification optical fiber.

The aforementioned configuration can decrease the number of modes havinghigh intensity in the outer layer among propagation modes of light inthe bridge fiber. Therefore, the return light entering the bridge fiberfrom the amplification optical fiber is coupled more to a mode havinghigh intensity in the core layer, so that the return light entering thebridge fiber from the amplification optical fiber can be mostly trappedin the core layer.

As a result, the propagation of the return light to the pumping lightsource through the pumping light inputting optical fiber can besuppressed without using a special member such as a reflective film.

Accordingly, there is implemented the bridge fiber, the combiner, andthe fiber laser device that can reduce the failure of the pumping lightsource caused by the return light.

It is preferred that the substance be the atmosphere.

There is no propagation mode of light passing outside the outer layerwhere the atmosphere surrounds the outer layer, so that the return lightentering the bridge fiber from the amplification optical fiber iscoupled more to a mode having high intensity in the core layer. Thereturn light entering the bridge fiber from the amplification opticalfiber can be further trapped in the core layer as a result.

It is also preferred that the substance be an outermost layer coveringthe outer peripheral surface of the outer layer.

When the outermost layer covers the outer layer, the impact on the outerlayer from outside can be reduced while much of the return light iscoupled to a mode having high intensity in the core layer, therebyretaining the function of trapping the return light and at the same timeincreasing the resistance of the bridge fiber.

Moreover, it is preferred that the index of refraction is lower in theoutermost layer than in the core layer.

In a case where the outer layer is covered by the outermost layer havingthe index of refraction lower than that of the core layer and the outerlayer, the effective index of refraction is lower in a mode having highintensity in the outermost layer than in a mode having high intensity inthe core layer among the propagation modes of light in the bridge fiber.This means that, among the light being propagated in the core layer, thelight coupled to the mode having high intensity in the core layer is notreadily coupled to the mode having high intensity in the outer layer. Asa result, the return light entering the bridge fiber from theamplification optical fiber can be further trapped in the core layerwhere the outer layer is covered by the outermost layer.

Furthermore, it is preferred that the outer diameter of the core layerat the other end face of the bridge fiber be larger than the outerdiameter of a core of an optical fiber that is fused to the core layer.

In this case, the return light from the amplification optical fiber canbe trapped in the core layer without leaving any portion of it behind,whereby the failure of the pumping light source caused by the returnlight can be further reduced as compared to when the outer diameter ofthe core layer is smaller than or equal to the outer diameter of thecore of the amplification optical fiber.

It is further preferred that the bridge fiber includes a taperedportion, the diameter of which decreases from the side corresponding tothe one end face of the bridge fiber toward the side corresponding tothe other end face of the bridge fiber.

In this case, the bridge fiber can collect the pumping light and guideit to the amplification optical fiber so that the amplificationefficiency can be improved, the pumping light being input from theplurality of pumping light inputting optical fibers through the end faceof the untapered side of the bridge fiber.

According to the present invention, there can be provided the bridgefiber, the combiner, and the fiber laser device that can easily reducethe failure of the pumping light source caused by the return light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a fiber laser device according to afirst embodiment;

FIG. 2 is a diagram illustrating a combiner;

FIGS. 3A and 3B are diagrams illustrating a state of a bridge fiber cutorthogonally to a longitudinal direction of the bridge fiber;

FIG. 4 is a diagram illustrating the result of monitoring the intensitydistribution of return light that returns to the bridge fiber;

FIG. 5 is a diagram illustrating a combiner according to a secondembodiment; and

FIGS. 6A and 6B are diagrams illustrating a state of a bridge fiberaccording to the second embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will now be described in detailwith reference to the drawings.

(1) First Embodiment

Configuration of Fiber Laser Device

FIG. 1 is a diagram illustrating a fiber laser device 1 according to afirst embodiment. As illustrated in FIG. 1, the fiber laser device 1according to the embodiment is a resonant fiber laser that includes, asa main component, a plurality of pumping light sources 11, a combiner12, a first FBG (Fiber Bragg Grating) 13 serving as a first mirror, anda second FBG 14 serving as a second mirror.

The plurality of pumping light sources 11 emits pumping light and isformed of a laser diode, for example.

FIG. 2 is a diagram illustrating the combiner 12. As illustrated inFIGS. 1 and 2, the combiner 12 includes as a main component a pluralityof pumping light inputting optical fibers 20, a bridge fiber 30, and anamplification optical fiber 40.

The plurality of pumping light inputting optical fibers 20 is an opticalfiber which inputs the pumping light emitted from the pumping lightsource 11 into the bridge fiber 30, where the number of the pumpinglight inputting optical fibers provided is the same as the number of thepumping light sources 11 provided. Each of the pumping light inputtingoptical fibers 20 including a core 21 and cladding 22 that covers thecore 21 is a multi-mode fiber, for example.

The index of refraction of the core 21 is higher than that of thecladding 22. Note that the core 21 is formed of a material such asquartz, while the cladding 22 is formed of a material such as quartzdoped with fluorine or the like.

The bridge fiber 30 is an optical fiber which optically couples theplurality of pumping light inputting optical fibers 20 and theamplification optical fiber 40 and includes, as a main component, a corelayer 31 and an outer layer 32 that covers an outer peripheral surfaceof the core layer 31 without any gaps.

The core layer 31 and the outer layer 32 includes a tapered portion 33,the diameter of which decreases from the side corresponding to one endface of the bridge fiber 30 facing an end face of the pumping lightinputting optical fiber 20 toward the side corresponding to another endface of the bridge fiber 30 facing an end face of the amplificationoptical fiber 40. The one end face of the bridge fiber 30 is anuntapered end face (hereinafter referred to as a large-diameter endface) EF1, whereas the other end face of the bridge fiber 30 is atapered end face (hereinafter referred to as a small-diameter end face)EF2.

An area AR1 of the outer layer 32 at the large-diameter end face EF1 isan area to be optically coupled to an end face of a core of each of theplurality of pumping light inputting optical fibers 20. In theembodiment, the end face of the core of each of the plurality of pumpinglight inputting optical fibers 20 is fused to the area AR1 of the outerlayer 32 such that the end faces of the cores are equally spaced.

On the other hand, an area AR2 of the core layer 31 at thesmall-diameter end face EF2 is an area to be optically coupled to an endface of a core of the amplification optical fiber 40. The end face ofthe core of the amplification optical fiber 40 is fused to the area AR2of the core layer 31 in the embodiment.

FIGS. 3A and 3B are diagrams illustrating a state of the bridge fibercut orthogonally to a longitudinal direction of the bridge fiber 30. Asillustrated in FIGS. 3A and 3B, an index of refraction n₁ of the corelayer 31 is lower than an index of refraction n₂ of the outer layer 32.The outer layer 32 is encircled by the atmosphere having the index ofrefraction lower than the index of refraction n₂ of the outer layer 32.That is, the outer peripheral surface of the outer layer 32 is encircledby air cladding.

The core layer 31 is formed of a material such as quartz doped withfluorine (F) or the like which decreases the index of refraction, whilethe outer layer 32 is formed of a material such as pure quartz.

The amplification optical fiber 40 as illustrated in FIG. 2 includes acore 41 to which one kind or two or more kinds of active elements areadded, first cladding 42 which covers the outer peripheral surface ofthe core 41, second cladding 43 which covers the outer peripheralsurface of the first cladding 42, and a coating layer 44 surrounding thesecond cladding.

The active element here includes a rare earth element such as erbium(Er), ytterbium (Yb) or neodymium (Nd), or bismuth (Bi) that is not therare earth element, for example.

The index of refraction is higher in the core 41 than in the firstcladding 42, higher in the first cladding 42 than in the second cladding43, and lower in the second cladding 43 than in the coating layer 44. Interms of suppressing the refraction of light entering from the bridgefiber 30, it is preferred that the index of refraction in the firstcladding 42 be equal to that in the outer layer 32 of the bridge fiber30.

The outer diameter of the core 41 is smaller than or equal to an outerdiameter OD of the core layer 31 of the bridge fiber 30 where, asdescribed above, the area AR2 of the core layer 31 at the small-diameterend face EF2 of the bridge fiber 30 is fused to the end face of the core41. Note that the coating layer 44 near the end face of the core of theamplification optical fiber 40 is peeled off while the end face of thecore is fused to the bridge fiber 30.

The first FBG 13 is provided in a region of the amplification opticalfiber 40 closer to the bridge fiber 30 than the second FBG 14 and has astructure where a part with high index of refraction is repeated in afixed cycle along the longitudinal direction of the amplificationoptical fiber 40. This part is adjusted to reflect at least a part ofthe wavelength of light emitted by the pumped active element in theamplification optical fiber 40.

The second FBG 14 is provided in a region of the amplification opticalfiber 40 farther from the bridge fiber 30 than the first FBG 13 and hasa structure where a part with high index of refraction is repeated in afixed cycle along the longitudinal direction of the amplificationoptical fiber 40. This part is adjusted to reflect light having the samewavelength as that of the light reflected by the first FBG 13 at areflectance lower than that of the first FBG 13.

Operation and Effect

In the fiber laser device 1 according to the embodiment, the pumpinglight entering the bridge fiber 30 from the pumping light source 11through the pumping light inputting optical fiber 20 is propagated inthe outer layer 32 of the bridge fiber 30 and then enters the cladding42 of the amplification optical fiber 40.

The pumping light having entered the cladding 42 is propagated in thecladding 42 and the core 41 of the amplification optical fiber 40 andpumps the active element added to the core 41, whereby the activeelement emits spontaneous emission light having a specific wavelength.

The spontaneous emission light is amplified while being propagated inthe core 41 of the amplification optical fiber 40 back and forth betweenthe first FBG 13 and the second FBG 14, so that a part of the lightamplified transmits through the second FBG 14 and is emitted from anoutput end of the amplification optical fiber 40.

Now, there is a case where a part of the light amplified between thefirst FBG 13 and the second FBG 14 transmits through the first FBG 13and returns to the bridge fiber 30.

In this case, among the propagation modes of light in the bridge fiber30, the number of modes having high intensity in the outer layer 32 canbe decreased because the outer layer 32 is encircled by the atmospherehaving the index of refraction n₂ of the outer layer 32 higher than theindex of refraction n₁ of the core layer 31 and lower than the index ofrefraction n₂ of the outer layer 32 in the bridge fiber 30 of theembodiment.

As a result, the return light entering the bridge fiber 30 from theamplification optical fiber 40 is coupled more to the mode having highintensity in the core layer 31. Therefore, the return light entering thebridge fiber 30 from the amplification optical fiber 40 can be mostlytrapped in the core layer 31.

FIG. 4 illustrates the result of monitoring the intensity distributionof the return light returning to the bridge fiber 30. Here, it isassumed that the diameter of the core layer 31 at the small-diameter endface EF2 of the bridge fiber 30 is 30 μm, the diameter of the outerlayer 32 is 125 μm, and the diameter reduction rate of thesmall-diameter end face EF2 with respect to the large-diameter end faceEF1 is 33%. The index of refraction n₂ of the outer layer 32 is set 0.8%higher than the index of refraction n₁ of the core layer 31.

Moreover, it is assumed that the diameter of the core 41 in theamplification optical fiber 40 is 6 μm, the diameter of the firstcladding 42 is 125 μm, and the diameter of the second cladding 43 is 250μm where the index of refraction of the core 41 is set 0.15% higher thanthat of the first cladding 42 and 5% higher than that of the secondcladding 43.

It is further assumed that the active element added to the core 41 ofthe amplification optical fiber 40 is ytterbium, an LD light source withthe wavelength of 915 nm is employed as the pumping light source 11, andthe reflective wavelength of the first FBG 13 is 1064 nm.

A dotted line P1 corresponds to the outer periphery of the core layer31, and a dot-dashed line P2 corresponds to the outer periphery of theouter layer 32 as illustrated in FIG. 4, where the intensity of thereturn light is the highest in the core layer 31 according to theintensity distribution of the return light. In other words, the returnlight returning to the bridge fiber 30 is collected in the core layer 31of the bridge fiber 30, the core layer 31 having a function of trappingthe return light. Note that an intersection between a vertical line anda horizontal line in FIG. 4 corresponds to an entrance point of thereturn light.

Accordingly, the propagation of the return light to the pumping lightsource 11 through the pumping light inputting optical fiber 20 can besuppressed without using a special member such as a reflective film,when the return light enters the bridge fiber 30 of the embodiment fromthe amplification optical fiber 40.

The embodiment can therefore realize the bridge fiber 30, the combiner12, and the fiber laser device 1 that can easily reduce the failure ofthe pumping light source 11 caused by the return light.

Note that the pumping light is mainly propagated in the area of theouter layer when the index of refraction n₁ of the core layer 31 islower than the index of refraction n₂ of the outer layer 32. Therefore,there can be suppressed the increase in the amount of pumping lightabsorbed by the element added to the core 41 of the amplificationoptical fiber 40 in the vicinity of the input end of the amplificationoptical fiber 40. As a result, the amplification efficiency can beimproved by propagating the pumping light farther in the amplificationoptical fiber 40, thereby also inhibiting the end portion of theamplification optical fiber 40 from generating significantly more heatthan another portion.

The outer layer 32 of the embodiment is surrounded by the atmosphere. Inthis case, there is no propagation mode of light passing outside theouter layer 32, whereby the return light entering the bridge fiber 30from the amplification optical fiber 40 is coupled more to the modehaving high intensity in the core layer 31. The return light enteringthe bridge fiber 30 from the amplification optical fiber 40 can befurther trapped in the core layer 31 as a result.

Also in the embodiment, the outer diameter OD of the core layer 31 atthe small-diameter end face EF2 of the bridge fiber 30 is larger thanthe outer diameter of the core of the amplification optical fiber 40fused to the core layer. The return light from the amplification opticalfiber 40 can thus be trapped in the core layer 31 without leaving anyportion of it behind, whereby the failure of the pumping light source 11caused by the return light can be further reduced as compared to whenthe outer diameter of the core layer 31 is smaller than or equal to theouter diameter of the core of the amplification optical fiber 40.

Also in the embodiment, the core layer 31 and the outer layer 32 of thebridge fiber 30 includes the tapered portion 33, the diameter of whichdecreases from the side corresponding to the large-diameter end facetoward the side corresponding to the small-diameter end face. As aresult, the bridge fiber 30 can collect the pumping light and guide itto the amplification optical fiber 40 fused to the small-diameter endface EF2 on the tapered side so that the amplification efficiency can beimproved, the pumping light being input from the plurality of pumpinglight inputting optical fibers 20 fused to the large-diameter end faceEF1 on the untapered side.

(2) Second Embodiment

Now, a second embodiment suited for the present invention will bedescribed in detail with reference to the drawings. Note that theredundant description of a component identical or equivalent to that inthe first embodiment will be omitted by assigning the same referencenumeral to the component unless described otherwise.

FIG. 5 is a diagram illustrating a combiner 12 according to the secondembodiment. As illustrated in FIG. 5, the combiner 12 in the embodimentdiffers from the combiner 12 of the first embodiment in that anoutermost layer 34 covering the outer peripheral surface of an outerlayer 32 is further included in the combiner of the embodiment.

FIGS. 6A and 6B are diagrams illustrating a state of a bridge fiberaccording to the second embodiment. As illustrated in FIGS. 6A and 6B,an index of refraction n₃ of the outermost layer 34 is lower than theindices of refraction n₁ and n₂ of a core layer 31 and the outer layer32, respectively. The outermost layer 34 is formed of a material such asresin having an index of refraction lower than the indices of refractionn₁ and n₂ of the core layer 31 and the outer layer 32, or quartz towhich fluorine is added.

In a case where the outer layer 32 is covered by the outermost layer 34with the index of refraction n₃ lower than the indices of refraction n₁and n₂ of the core layer 31 and the outer layer 32, the effective indexof refraction is lower in a mode having high intensity in the outermostlayer 34 than in a mode having high intensity in the core layer 31 amongpropagation modes of light in a bridge fiber 30.

This means that, among the light being propagated in the core layer 31,the light coupled to the mode having high intensity in the core layer 31is not readily coupled to the mode having high intensity in the outerlayer 32. Return light entering the bridge fiber 30 from anamplification optical fiber 40 can be trapped in the core layer 31 as aresult.

Moreover, the outermost layer 34 being physically provided outside theouter layer 32 can reduce the impact on the outer layer 32 from outsideas compared to the first embodiment.

While the first and second embodiments have been described as anexample, the present invention is not limited to the aforementionedembodiments.

For example, the small-diameter end face EF2 of the bridge fiber 30 isdirectly fused to the one end face of the amplification optical fiber 40in the first and second embodiments. However, a relaying optical fibermay be interposed between the bridge fiber 30 and the amplificationoptical fiber 40 as long as the bridge fiber 30 is optically coupled tothe amplification optical fiber 40.

In such case where the relaying optical fiber is interposed, one endface of the relaying optical fiber is fused to the small-diameter endface EF2 of the bridge fiber 30, and another end face of the relayingoptical fiber is fused to one end face of the amplification opticalfiber 40. A double-cladding fiber can be applied as the relaying opticalfiber where a rare earth element is not added to a core of the fiber.Similar to the first and second embodiments, it is preferred that theouter diameter of the core layer 31 of the bridge fiber 30 be largerthan the outer diameter of the core of the relaying optical fiber.

Moreover, the tapered portion 33 provided to the bridge fiber 30 in thefirst and second embodiments may be omitted.

While six of the pumping light inputting optical fibers 20 are providedas an example in the first and second embodiments, the number of pumpinglight inputting optical fibers may be anywhere between two to fivepieces or seven or more pieces. In short, various numbers of the pumpinglight inputting optical fibers can be applied as long as two or more ofthem are provided. Note that, in terms of homogenizing the opticalproperty caused by the arrangement of the pumping light inputtingoptical fibers 20, it is preferred to apply the number of pumping lightinputting optical fibers such that the fibers can be arrangedsymmetrically about the central axis of the core layer 31 in the bridgefiber 30.

While the outer diameter OD of the core layer 31 at the small-diameterend face EF2 of the bridge fiber 30 is larger than the outer diameter ofthe core of the amplification optical fiber 40 fused to the core layer31 in the first and second embodiments, the outer diameter OD may besmaller than or equal to the outer diameter of the core.

Furthermore, the index of refraction n₃ of the outermost layer 34 in thesecond embodiment is lower than the indices of refraction n₁ and n₂ ofthe core layer 31 and the outer layer 32. The index of refraction n₃ ofthe outermost layer 34 may however be higher than the index ofrefraction n₁ of the core layer 31 as long as the index of refraction n₃is lower than the index of refraction n₂ of the outer layer 32.

Note that in addition to what is illustrated in the first and secondembodiments or another embodiment, each component of the bridge fiber30, the combiner 12, and the fiber laser device 1 described above can becombined, omitted, modified, or subjected to addition of a knowntechnique as appropriate without departing from the object of thepresent application.

The invention claimed is:
 1. A bridge fiber comprising: a core layer;and an outer layer which has an index of refraction higher than an indexof refraction of the core layer and covers an outer peripheral surfaceof the core layer, wherein the outer layer is entirely surrounded by anatmosphere having an index of refraction lower than the index ofrefraction of the outer layer, an area of the outer layer at one endface of the bridge fiber is an area that is to be optically coupled toan end face of a core of each of a plurality of pumping light inputtingoptical fibers, and an area of the core layer at the other end face ofthe bridge fiber is an area that is to be optically coupled to an endface of a core of an amplification optical fiber.
 2. The bridge fiberaccording to claim 1 wherein an index of refraction of the atmosphere islower than the index of refraction of the core layer.
 3. The bridgefiber according to claim 1, wherein an outer diameter of the core layerat the other end face of the bridge fiber is larger than an outerdiameter of a core of an optical fiber fused to the core layer.
 4. Thebridge fiber according to claim 1, further comprising a tapered portion,a diameter of which decreases from a side corresponding to the one endface of the bridge fiber toward a side corresponding to the other endface of the bridge fiber.
 5. A combiner comprising: a plurality ofpumping light inputting optical fibers; and a bridge fiber including: acore layer; and an outer layer which has an index of refraction higherthan an index of refraction of the core layer and covers an outerperipheral surface of the core layer, wherein the outer layer isentirely surrounded by an atmosphere having an index of refraction lowerthan the index of refraction of the outer layer, an area of the outerlayer at one end face of the bridge fiber is optically coupled to an endface of each of the plurality of pumping light inputting optical fibers,and an area of the core layer at the other end face of the bridge fiberis an area that is to be optically coupled to an end face of a core ofan amplification optical fiber.
 6. A fiber laser device comprising: aplurality of pumping light inputting optical fibers; an amplificationoptical fiber; a bridge fiber including: a core layer; and an outerlayer which has an index of refraction higher than an index ofrefraction of the core layer and covers an outer peripheral surface ofthe core layer; and a pair of mirrors which is disposed at apredetermined distance from each other in the amplification opticalfiber, wherein the outer layer is surrounded by a substance having anindex of refraction lower than the index of refraction of the outerlayer, an area of the outer layer at one end face of the bridge fiber isoptically coupled to an end face of each of the plurality of pumpinglight inputting optical fibers, and an area of the core layer at theother end face of the bridge fiber is optically coupled to an end faceof a core of the amplification optical fiber.
 7. The bridge fiberaccording to claim 2, wherein an outer diameter of the core layer at theother end face of the bridge fiber is larger than an outer diameter of acore of an optical fiber fused to the core layer.
 8. The bridge fiberaccording to claim 2, further comprising a tapered portion, a diameterof which decreases from a side corresponding to the one end face of thebridge fiber toward a side corresponding to the other end face of thebridge fiber.
 9. The fiber laser device according to claim 6, whereinthe substance is atmosphere.
 10. The fiber laser device according toclaim 6, wherein the substance is an outermost layer covering an outerperipheral surface of the outer layer.
 11. The fiber laser deviceaccording to claim 10, wherein an index of refraction of the outermostlayer is lower than the index of refraction of the core layer.
 12. Thefiber laser device according to claim 6, wherein an outer diameter ofthe core layer at the other end face of the bridge fiber is larger thanan outer diameter of a core of an optical fiber fused to the core layer.13. The fiber laser device according to claim 6, further comprising atapered portion, a diameter of which decreases from a side correspondingto the one end face of the bridge fiber toward a side corresponding tothe other end face of the bridge fiber.